The present invention relates to the production process of biotin by fermentation using a genetically engineered organism.
Biotin is one of the essential vitamins for nutrition of animals, plants, and microorganisms, and very important as medicine or food additives.
Biotin biosynthesis of Escherichia coli has been studied well, and it has been clarified that biotin is synthesized from pimelyl CoA via 7 keto-8-amino pelargonic acid (KAPA), 7,8-diamino pelargonic acid (DAPA) and desthiobiotin (DTB) [Escherichia coli and Salmonella typhimurium, Cellular and Molecular Biology, 544, (1987)]. The analysis of genetic information involved in the biosynthesis of biotin has been advanced on Escherichia coli [J. Biol. Chem., 263, 19577, (1988)] and Bacillus sphaericus (U.S. Pat. No. 5,096,823). At least four enzymes are known to be involved in this biosynthetic pathway. These four enzymes are encoded by the bioA, bioB, bioD and bioF genes. The bioF gene codes for KAPA synthetase which catalyzes the conversion of pimelyl CoA to KAPA. The bioA gene codes for DAPA aminotransferase which converts KAPA to DAPA. The bioD gene codes for DTB synthetase which converts DAPA to DTB. The bioB gene codes for biotin synthase which converts DTB to biotin. It has been also reported that the bioC and bioH genes are involved in the synthesis of pimelyl CoA in Escherichia coli.
There are many studies on fermentative production of biotin. Escherichia coli (Japanese Patent Kokai No. 149091/1986 and Japanese Patent Kokai No. 155081/1987), Bacillus sphaericus (Japanese Patent Kokai No. 180174/1991), Serratia marcescens (Japanese Patent Kokai No. 27980/1990) and Brevibacterium flavum (Japanese Patent Kokai No. 240489/1991) have been used. But these processes have not yet been suitable for use in an industrial production process because of a low productivity. Moreover, large amounts of DTB, a biotin precursor, accumulates in the fermentation of these bacteria. Therefore, it has been assumed that the last step of the biotin biosynthetic pathway, from DTB to biotin, is a rate limiting step.
On the other hand, it was found that a bacterial strain belonging to the genus Kurthia produces DTB and small amounts of biotin. Also mutants which produce much larger amounts of biotin were derived from wild type strains of the genus Kurthia by selection for resistance to biotin antimetabolites acidomycin (ACM), 5-(2-thienyl)-valeric acid (TVA) and alpha-methyl desthiobiotin (MeDTB). However, in view of the still low biotin titers it is desirable to apply genetic engineering to improve the biotin productivity of such mutants.
The present invention relates therefore to the chromosomal DNA fragments carrying the genes involved in the biotin biosynthesis of Kurthia sp. The isolated chromosomal DNA fragments carry 8 genes, the bioA, bioB, bioC, bioD, bioF, bioFII, bioH and bioHII genes, and transcriptional regulatory sequences. The bioFII gene codes for an isozyme of the bioF gene product. The bioHII gene codes for an isozyme of the bioH gene product.
The present invention further relates to Kurthia sp. strains in which at least one gene involved in biotin biosynthesis is amplified, and also to the production process of biotin by this genetically engineered Kurthia sp. strain.
Although the DNA fragment mentioned above may be of various origins, it is preferable to use the strains belonging to the genus Kurthia. Specific examples of such strains include, for example, Kurthia sp. 538-6 (DSM No. 9454) and its mutant strains by selection for resistance to biotin antimetabolites such as Kurthia sp. 538-KA26 (DSM No. 10609).